Illustration of the equality of PMa modulo Ω between two systems with the same central star and a companion on a long-period orbit but with different values of sma and mass. For a given sma and a given mass of the companion (left panel) the PMa is directed toward the companion. There exists a smaller sma and a larger mass for which the ‖PMa‖ is the same (right panel) but the orientation at equal Ω is different. Nevertheless, it is possible to align the PMa on the same position angle (PA) by rotating the system by some ΔΩ.